Proposal Summary: The androgen receptor (AR), a member of the nuclear receptor family of ligand activated transcription factors, is a key regulator of the processes involved in the regulation of the male reproductive tract and in the development and maintenance of muscle. In addition however, inappropriate signaling by this receptor is implicated in the pathogenesis of prostate cancer (1,2). Thus, although androgens have clinical utility for the treatment of hypogonadism and cachexia, the untoward side effects with respect to prostate function have limited their use. This finding highlights the need for compounds whose androgenic actions are manifest in a tissue selective manner. Compounds of this class, now referred to as Selective Androgen Receptor Modulators (SARMs) were not comtemplated until recently. It is now considered that tissue/process selective androgens will be developed by exploiting the ability of ligands to facilitate the preferential interaction of the receptor with one cofactor over another (3,4). Whereas the majority of work in the field to date has focused on defining the roles of AR-cofactors in the process of.gene activation it has become clear to us that an examination of the roles of cofactors in negatively regulated gene transcription is lacking. Thus we have used (a) classical gene array studies to identify several hundred genes whose expression is downregulated by androgens in prostate cancer cells, (b) high throughput protein-protein interaction screens to isolate 265 proteins that interact with ligand activated AR and (c) chemical screens to define small molecules that permit the differential recruitment of cofactors to AR. With these key tools in hand we are in a good position to begin to define the cofactors that are important for AR- mediated target gene repression. To complete these goals, we propose the following specific aims: Aim 1: Definition of the mechanistic attributes of an AR-ligand that enable it to suppress gene transcription Aim 2: Identification of the cofactors required for AR-mediated gene repression Aim 3: Identification and comparative analysis of AR and cofactor recruitment to AR repressed genes Relevance: It is estimated in 2006, 234,460 men will be diagnosed with and 27,350 men will die from prostate cancer. Thus, due to the role of androgen receptor (AR) in prostate cancer, the identification of prostate cancer treatments and preventatives that can uncouple the positive effects from the negative effects of AR are of high necessity. The studies proposed within will assist in such discoveries.